Directory of Mark Twain's maxims, quotations, and various opinions:

LETTERS

When you get an exasperating letter what happens?
If you are young, you answer it promptly, instantly--and mail the thing
you have written. At forty what do you do? By that time you have found out
that a letter written in passion is a mistake in ninety-nine cases out of
a hundred.
- Mark Twain, a Biography

An old, cold letter ....makes you wonder how you could ever have got into
such a rage about nothing.
- Mark Twain, a Biography

From
the Dave Thomson collection

The most useful and interesting letters we get here from home are from children
seven or eight years old...They write simply and naturally and without straining
for effect. They tell all they know, and stop.
- "An Open Letter to the American People," New York Weekly Review,
17 February 1866

The reason I dread writing letters is because I am so apt to get to slinging
wisdom & forget to let up. Thus much precious time is lost.
- Letter to James Redpath, 15 June 1871

The following quote is often misattributed to Mark Twain:

"I have made this letter longer than usual, only because I have
not had time to make it shorter." This quote is by the 17th-century
French philosopher and mathematician, Blaise Pascal (1623-62),
written in a letter to a friend. The original French version was: "Je
n'ai fait cette lettre - ci plus longue que parce que je n'ai pas eu le
loisir de la faire plus courte"